At present, “Automated Mechanical Transmissions” or AMTs are becoming increasingly common in heavy-duty vehicles. With a view to determining the gear selection strategy for vehicles with AMT, an estimate or calculation is generally made of the current road resistance. The current road resistance is determined on the basis of input signals from a number of sensors, after which an electronic control unit determines a suitable gear selection strategy.
For the selection of a starting gear, this generally means that the control unit only pays heed to the weight of the vehicle and the gradient of the ground surface, whereupon it selects a starting gear offering optimal starting characteristics. This normally means that a relatively high starting gear is selected and that the transmission changes up as soon as the gear selection strategy allows. The intention is for the vehicle to be able to be started and reach cruising speed as quickly as possible.
This gear selection is not ideal on all occasions. For example, in the tipping of cargo from a platform, in tipping-related marshalling or in low-speed maneuvering at the same time as some body device on the vehicle is being used, a starting gear selected in the aforesaid manner may be unsuitable.
Through EP 1 1 03 742, for example, an AMT system can be seen in which the driver can manually select a preselected starting gear provided that this selection is accepted by the control unit of the transmission. If a power take-off is activated, the control unit will temporarily switch from automatic to manual gear mode.
This system calls on the driver to make certain settings with respect to the starting gear. In the event of a change of driver, problems may also arise if a previous driver has preselected a starting gear which is not suitable for the maneuver about to be performed.
U.S. Pat. No. 5,455,767 describes a way of controlling the transmission if the vehicle is in motion when the power take-off is activated. By detecting the particular power consumption of the power take-off, it is possible for the control unit of the transmission to move the gearshift points of the gear selection strategy. The method does not concern selection and/or locking of a starting gear under certain conditions. Moreover, the method requires the control unit to change to a manual transmission mode to allow the driver to select a starting gear.
An object of the invention is therefore to solve the aforementioned problems by offering a method in which the control unit of the transmission detects that a power take-off is active when starting from stationary or almost stationary, a preprogrammed starting gear being selected and maintained until at least one predetermined condition has been met, for example that the power take-off is deactivated. This occurs without the automatic gearshift mode of the vehicle being disengaged and without the driver needing to intervene.